Inflation with vector fields revisited: non-Gaussianities

Abstract

{We revisit the resulting bispectrum of inflation with kinetic-coupled vector fields by organizing the dynamics in terms of $h$, which measures the vector kinetic contribution relative to that of the scalar field. We derive a low-energy effective field theory (EFT) and evaluate the bispectrum of the curvature perturbation for the large-$h$ regime. For $h\gg 1$, the entropic perturbation becomes heavy and can be integrated out; the remaining curvature mode has an imaginary sound speed and undergoes transient growth before horizon crossing. In contrast to the $h\ll 1$ regime, where transfer from the vector sector persists outside the horizon and produces a local-type contribution enhanced as $h^2{N}_K^3$, we find that the bispectrum signals are controlled by the matching parameter $x\simeq 1.63Ah$, where $A<1$ is a numerical factor remaining dependent on the UV-to-EFT matching prescription. As a result, in addition to the known flattened-enhanced signals scaling as $x^3$, a flattened-enhanced signal scaling as $x^2$ and a pronounced local projection scaling as $x$ are present. The bispectrum therefore distinguishes vector-supported inflationary dynamics even for an exactly isotropic background.

32 pages, 9 figures, 2 appendix; Publication version